Modern automotive ignition systems are increasingly dependent upon operational and diagnostic information to control internal combustion engine operation. One of the difficult challenges facing designers of modern engine control systems is to use such information to provide for satisfactory engine performance while also maintaining engine emissions below increasingly aggressive emissions standards mandated by government agencies. One particular concern in providing such control is the quality of combustion of the air-fuel mixture in the individual cylinders of the engine. Absent or incomplete combustion not only compromises engine performance, but also represents a significant pollution hazard.
A number of factors are known to contribute to poor combustion quality including, for example, misfire and fouled ignition plug conditions. Diagnostic systems for detecting combustion related problems such as misfire or fouled plug conditions are known and rely on various engine operating conditions to discriminate fault or failure conditions. One such known diagnostic system is operable to detect combustion as well as other engine performance parameters as a function of ion current flowing across the ignition plug electrode gap. However, such ion current-based systems are typically limited to determining only whether per-cylinder combustion has, or has not, occurred. What is needed is an ion-current based system for determining not only whether combustion has occurred, but for further determining the quality of the combustion event if it does occur and for diagnosing the overall health of the ignition plug.